1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly to a method and an apparatus for compensating the white balance of a plasma display panel in order to improve its picture quality.
2. Description of the Related Art
A plasma display panel PDP displays a picture in use of the visible ray generated from a phosphorus material when the phosphorus material is excited by the ultraviolet ray generated by a gas discharge. The PDP has advantages that it is thinner and lighter than a cathode ray tube CRT, which has been a display means most widely used so far, and it is possible to be made into a high definition screen and bigger in size.
The PDP is driven by dividing one frame into several sub-fields that have different light emission frequency, for realizing the gray level of a picture. Each sub-field can be divided again into a reset period for generating a uniform discharge, an address period for selecting discharge cells and a sustain period for realizing gray levels in accordance with a discharge frequency For instance, in the event that it is wanted to display a picture with 256 gray levels, a frame period 16.67 ms corresponding to 1/60 second is divided into 8 sub-fields. In addition, each of 8 sub-fields is divided again into the reset period, the address period and the sustain period. Herein, the reset period and the address period of a sub-field are the same for each sub-field, but on the other hand, the sustain period and the discharge frequency thereof increase in proportion of the number of sustain pulses at the rate of 2n (n=0,1,2,3,4,5,6,7) in each sub-field. In this way, since the sustain period becomes different in each sub-field, it is possible to realize the gray level of a picture.
There is included a circuit that compensates white balance for increasing display quality in this PDP.
Referring to FIG. 1, a conventional PDP driving circuit includes gamma correctors 2R, 2G and 2B each receiving video data of red, green and blue, and gain controllers 4R, 4G and 4B, error diffusers 6R, 6G and 6B and sub-field mapping units 8R, 8G and 8B connected between the gamma correctors 2R, 2G and 2B and a data driver of the PDP.
The gamma correctors 2R, 2G and 2B apply reverse gamma correction to the video data of red, green and blue to linearly convert brightness values in accordance with gray level values of the video data. To this end, the gamma correctors 2R, 2G and 2B apply gamma correction to the data of red, green and blue by equally raising them to the 2.2 power. The ratio of the red, green and blue data is set to be 0.8:1:1.2 to have correct white balance.
                              γ          ⁡                      (                          R              ,              G              ,              B                        )                          =                                            (                                                input                  ⁢                                                                          ⁢                  data                                255                            )                        2.2                    .                                    Formula        ⁢                                  ⁢        1            
The gain controllers 4R, 4G and 4B adjust gains by multiplying values by the red, green and blue video data corrected by the gamma correctors 2R, 2G and 2B, wherein the values are set in advance at the rate of white balance as in FIG. 3.
The error diffusers 6R, 6G and 6B finely adjust brightness values by diffusing error components to adjacent cells with respect to the data from the gain controllers 4R, 4G and 4B. To this end, the error diffusers 6R, 6G and 6B separate the data into integer parts and fractional parts, and multiply the fractional parts by Floy-Steinberg coefficient to diffuse the error components to the adjacent cells thereto.
The sub-field mapping units 8R, 8G and 8B map the data inputted from the error diffusers to a sub field pattern that is set in advance so as to apply them to a data aligner 12.
The data aligner 12 stores the video data, which are inputted from the sub field mapping units 8R, 8G and 8B, at a memory 10, and at the same time, retrieves the data stored at the memory 10 to apply to the data driver of the PDP (not shown).
The data driver of the PDP is implemented as integrated circuits IC each connected to each of a plurality of data lines formed in the PDP, and applies the data inputted from the data aligner 12 to the data lines of the PDP.
FIG. 4 illustrates another driving circuit of a conventional PDP.
Referring to FIG. 4, the driving circuit of the conventional PDP includes an average picture level APL controller 20 for detecting the average brightness of input images per frame, and a gain controller 4 for adjusting gains with respect to the red, green and blue video data inputted from the gamma corrector 2A in accordance with the APL detected by the APL controller 20.
The APL detected by the APL controller 20 is inputted to the gain controller 4 and to a timing controller (not shown) at the same time. The timing controller controls a circuit to have the number of sustain pulses adjusted, wherein the circuit generates the sustain pulses.
The frame memory 14 acts to delay the data inputted from an input line for one frame period and to apply them to the gamma corrector 2A. With regard to the gamma corrector 2A and 2B, the error diffuser 6, the sub-field mapping unit 8, the data aligner 12 and memory 10, they substantially have the same functions as those shown in FIG. 1, thus their detail description will be omitted.
By the way, because the saturation characteristic of red, green and blue phosphoruses, which vary with discharge frequency, is not considered in the conventional PDP driving circuit, there is a problem that desired colors are not expressed in accordance with the APL or the discharge frequency.
To describe this in detail, the composition of red phosphorus generally used in the PDP is YgdBO3:Eu3+, the composition of green phosphorus is Zn2SiO4:Mn2+, and the composition of blue phosphorus is BaMgA1110017:Eu2+. According to the experiment result, the saturation characteristics of the red, green and blue phosphoruses in accordance with the discharge frequency as in FIG. 5 come to be different. In other words, the blue phosphorus has its brightness proportional to the discharge frequency, the red phosphorus has its brightness proportional to about (discharge frequency)0.9, and the blue phosphorus has its brightness proportional to about (discharge frequency)0.85. In this way, the brightness saturation characteristic of the blue phosphorus is linear to the discharge frequency, but the brightness saturation characteristics of the red phosphorus and green phosphorus are non-linear to the discharge frequencies. Accordingly, if in theory, gamma correction is applied to the red, green and blue video data by raising them to the 2.2 power, the gray level values should be expressed as normal. However, in practice, the gray levels can be expressed only when the gamma correction is applied to each of the red, green and blue data with their optimal values due to the different saturation characteristic of phosphorus by red, green and blue.
As described above, the saturation characteristic of phosphorus is different by red, green and blue, thus the white balance in accordance with the discharge frequency is not the same. For instance, yellowish white color appears if the discharge frequency is several hundreds when displaying white color, and bluish white color appears if the discharge frequency is increased to several thousands. This problem cannot be solved in the conventional driving circuit. In other words, the reference values of the gain adjustment and gamma correction set in the PDP driving circuit as in FIG. 1 are fixed to the values for which the discharge frequency was not taken into consideration. In the driving circuit of the PDP as in FIG. 4, the gains of the red, green and blue video data are adjusted in a fixed rate of the white balance that is set in advance regardless of the discharge frequency even though the gains are adjusted in accordance with the APL value. More specifically, in the event that the number of sustain pulses is adjusted in accordance with the APL value, if the number of sustain pulses is decreased to a few hundreds in a Full White Pattern where the full screen is displayed in white as in FIG. 6A, the yellowish white color appears according to the different saturation characteristics of the red, green and blue phosphoruses, and the bluish white color appears in a white color area if the number of sustain pulses is increased to several thousands in a Window Pattern where part of the screen is displayed in white as in FIG. 6B.